Art of reclaiming plastic scrap



Patented May 17, 1949 ART OF BECLAIMING PLASTIC SCRAP Isador Miller, NewYork, and Arthur L. Bclser,

Jackson Heights, N. Y" signers to Gem Participations, Inc., New York, N.Y., a corporation of New York No Drawing. Application June 27, 1945.

Serial No. 601,94

4 Claims. (Cl. 260-486) The present invention relates to thereprocessing of plastic scrap produced from sheeting, coating. adhesive,injection molding compound or casting resin, more especially where theplastic scrap is of type derived from monomers containing the vinylgroup, for the purpose of recovering therefrom monomer that hassubstantially the same characteristics and utility as that from whichthe plastic was originally prepared.

While the invention has an important application to the reprocessing ofsynthetic resins that are thermoplastic only in a limited sense, such asmethyl methacrylate ("Lucite) scrap, particularly such scrap derivedfrom casting or machining, it lends itself also to the recovery ofmonomers from thermoplastic polymer, i. e., vinyl derivatives generally,such as vinyl acetate, vinyl chloride, vinyl benzene (styrene),chloride, acrylates and methacrylates generally, as well as to vinylcopolymers, such for instance. as copolymers of vinyl acetate and vinylchloride. butacite (vinyl butyraldehyde derivative) and mixedmethacrylate butacite scrap, all of which are derived from monomerscontaining-the vinyl group and may he considered as derivatives thereof.

It is among the objects of the invention to Provide a decompositionprocess for expeditiously and efliciently depolymeriaing scrap of thecharacter noted, that is, heterogeneous mixtures of basically identicalcompounds of different molecular weights for a large yield of pure waterwhite monomer and the recovery of valuable plasticisers from theresidues, which process is easy to control, simple to operate andinherently avoids the tendency to repolymerize the monomer in the finalstep of isolating and purifying the same.

The process involves generally pyrolysis, that is, decomposition by theapplication of heat to the scrap while immersed in a liquid internalheat transfer medium that is itself volatile without decomposition. Thepyrolysis is preferably conducted under reduced pressure and attemperatures considerably lower than that required where solid inertheat transfer media, such as sand or molten metal, are used. Indeed, bythe present invention much of the heating is effected at temperaturessubstantially below the temperature of decomposition of the polymermass.

The liquid internal heat transfer medium is stable. that is. ofcharacter to volatilize without decomposition under the temperature andpressure conditions of the process. It also has the property of wettingthe scrap and is or specific gravity lower than that of the scrap beingtreatvlnylidene ed. so that the heat transfer medium readily and easilycarries the heat uniformly to all portions of the scrap under treatment.

The liquid internal heat transfer medium has a boiling point much higherthan that of the monomer to be recovered as the end product. Thecomposition of the vapors will depend on the boiling range of the heattransfer medium, as well as the proportion of monomer present therein atany given instant. As a. general rule, the heat transfer medium willform the larger portion of the vapor phase. While the heat transfermedium need not have a solvent action either upon the polymer or uponits various products of depohrmerization, it is within the scope of theinvention to use a heat transfer liquid which has a solvent action onsome or all of such products.

Examples of heat transfer liquid that have but little solvent actionupon the polymer scrap but have a substantial solvent action on some ofits depoiymerization products, are dimethyl maleatc and dibutylphthalate, or mixtures of the two.

Since the boiling point of dibutyi Dhthalate, for example, in thepreferred pressure range indicated below is lower than the temperaturesrequired to! the most refractory polymers. it is obvious that dibutylphthalate alone can be used only for polymers decomposable in thatrange.

A preferred heat transfer medium that has a substantial solvent actionboth upon the scrap and upon its various depolymerization products is aliquid consisting of the crude complex of low molecular weight compoundsderived from the scrap itself and recovered as a residue in thepurification of monomer by fractional distillation at atmosphericpressure. In a preferred embodiment of the present invention, we use thecrude complex recovered as residue in the purification of methylmethacrylate monomer.

Any non-reactive plasticiser originally present in the polymer scrapunder treatment will, of course. be recovered as such during the heattreatment and will remain as a constituent of the crude complexrecovered as residue after purification of the monomer. We have alsofound that this residue will contain substances not originally presentin the polymer, and which apparently are products of the pyrolysis. Forexample, we have identified dimethyl maleate as a product ofdecomposition of methyl methacrylate polymer.

The scrap being treated should be of weicht approximately equal to theweight of the heat transfer medium at the beginning and as the pyrolysisproceeds, the amount of material functimingasheattransfermediumincreasesbeeameitisoneoithepmductsoi'reaction.

The heat transfer medium chosen in any giveninstaneewilldependonitspropertleflinrelation tothepolymertobetreated.Itmustbe liquid. stable, scrap wetting and volatile withoutdecomposition. Its specific gravity must be equal to orlessthanthatofthescrap. Itmustboilat temperatures equal to or higher than the operatingtemperatures at the given We have discovered that the use of the complexset forth avoids objectionable discoloration of the monomer yield. Thecolor bodies commonly carried over with the monomer vapor are eitherretained by or destroyed in the crude complex remaining as still residuein the submuent purification and recovery of the monomer by fractionaldistillation at atmospheric pressure. No color adsorbents or bleachingor other chemical p r fy s a ents are required.

We have also discovered that said complex in small amounts act as apolymerization inhibitor and when present to the extent of substantiallymorethantenpercent actsasapolymerization preventative. Since the complexas noted. is always present in the still in percentage several times ten(10%) per cent, polymerization during purification is effectively andcompletely avoided. No foreign solid polymerization inhibitor such ashydroquinone is required with its objectionable tendency to sublime anddiscolor the monomer during distillation.

Bearing in mind that the polymerized product to be processed is not achemical entity of definite composition but a heterogeneous mixture ofcompounds of different molecular weights but basically identicalstructure, there is no one temperature of decomposition, but in generaldecomposition or depolymerisation is progressive. Apparently compoundsof a given degree of powmerization are decomposed more readily, that is,under less severe conditions of heat and pressure than compounds ofother degrees of polymerisation. Therefore, it is preferred inoperation, progressively to increase the temperature orprogresslvelytoreducethepressureorboth,soas to depolymerize the morereactive polymer molecules at temperature well below the decompositiontemperature of the mass, and progressively to depolymerize the morerefractory Poly er molecules as the intensity of operation (increase oitemperature, decrease of pressure) is enhanced. The most refractory oi'the molecules require a temperature well above the tempera- 55 ture ofdecomposition of the polymer mass as such. Of course, the temperaturecould belimited,ifdesired,toanymaximumshortofcompletepyrolysisofthepolymermasawithcorrespondinglyreduced yield.

In the treatment. for instance, of methyl methacrylate scrap of thegeneral type known as Lacite" in our preferred heat transfer bath.although the reaction starts at even lower temperature, a practical rateof decomposition is obtained at a temperature of 200 degrees C. which iswell below the temperature of decompositionofthescrapassuch,underapressurenotexceeding45to50mm.,alth0uzhfldesired,thatpressuremaybeashighas'l5mm.ifthetemperatureisincreasedsomewhat,sayto225degrees 0. As depolymerisation w the temperature israised pr gr ssively and if esired or convenient, the pressure may belowered concomitantly. Bmhloweringofpressureisaconvenient way ofcontrolling the temperature and avoiding objectionable overheating. Themaximum temperature to which the scrap is raised is 50 degrees C. butpreferably that temperature should not be permitted to rise above 340degrees C. The operating w; a in the course of operation should neverexceed 120 mm. but maybecomesslow as 10 mm. with arangeof 50 to 25 mm.as the practical optimum. Within the range specified, about one-half ofthe scrap will be d below 300 degrees C. and the remainder above.

The vapor derived from the process is condensed in a suitably chilledcondenser and the pure monomer may be recovered from the condensate byfractional distillation at atmospheric pressure. As above indicated. thestill residue from this purification ordinarily consists of a mixture ofthe added heat transfer medium, any plasticiser (as dibutyl phthalateandthelike) originally present in the scrap and the crude complea of lowmolecular compounds resulting from the pyrolysis by molecularre-arrangement.

A specific example of the foregoing process as applied to thedepolymerisation of a typical batch of methyl methacrylate scrap is thefollowing: 600 parts of clean methyl methacrylate ("Lucite") scrap and800 parts of the crude low molecular complex above referred to, areheated at 48 to 50 mm. pressure in the range progressively rising from200 to 340 degrees C. The distillation condensed at 5 to l0 C. arecollected at various temperatures as follows:

Parts To 200' C 158 201 to 250" C 132 251' to 300 C 220 301' to 340 623On purification by fractional distillation at atmospheric pressure, 375parts of pure water-white methyl methacrylate monomer are recovered.

The still residue (758 grams) will contain approximately 40-42 grams ofmonomer which can be re-treated in a subsequent batch. Taking intoconsideration the 600 grams heat transfer medium originally added, about110-120 grams crude residues will be obtained. In addition toapproximately 25-30 grams of dibutyl phthalate apparently originallypresent as plasticizer, about 80 grams residue are obtained as a resultof the It will be apparent that the operation could be conducted in abatch or in a continuous process. In the latter procedure, suitableprovision is made for fractionation of the vapors and refluxing back adefinite proportion of the condensed and separated heat transfer medium,as fresh scrap is added in Pr portion.

Where the depolymerizationproducts are predominantly gaseous at ordinarytemperatures and pressures, as for example in the case of certain vinylcopolymers, provision is made for collectins the products at suitablelow temperatures ,modified pressures.

It will of course be understood by those skilled in the art, thatoptimum conditions of temper-' ature and pressure will vary with thechemical nature of the polymer being treated and the depolymerizationproducts to be derived.

Having thus described our invention, what we claim as new and desire tosecure by Letters Patent of the United States is:

1. Process for utilising polymer scrap derived auasu from monomercontaining the vinyl group which consists in decomposing the scrap bypyrolysis at subatmospheric pressures, and progressively increasingtemperatures with said scrap immersed in a stable volatile liquidsolvent internal heat transfer medium comprising a crude complex oi lowmolecular weight compounds of specific gravity lower than that of thescrap and which is recovered as a residue in the purification ofmonomers by the fractional distillation of the products of pyrolyticdecomposition of such polymer scrap.

2. In the process for utilizing methyl methacrylate scrap the step ofsubjecting the scrap to pyrolysis while immersed in a stable volatileliquid solvent internal heat transfer medium, comprising a crude complexof low molecular weight compounds of specific gravity lower than that ofthe scrap and which is recovered as a residue in the purification ofmonomers by the fractional distillaiton or the products of pyrolyticdecomposition of such polymer scrap, first at temperatures below thedecomposition temperature of the scrap as such, and thereafter atprogressively higher temperatures, all under reduced pressure in therange of 120 mm. to mm.

3. In the process of utilizing methyl methacrylate scrap, the pyrolysisthereof while immersed in a stable volatile liquid solvent internal heattransfer medium consisting essentially 01' a crude complex of lowmolecular weight compounds of specific gravity lower than that of thescrap and which is recovered as a residue in the purification ofmonomers by the fractional distillation oi the products of pyrolyticdecomposition of such polymer scrap, said pyrolysis being conducted atan elevated temperature progressively rising in the range between 200degrees and 350 degrees C. and at reduced pressure in the range of 120mm. to 10 mm.

4. In the process of utilizing methyl methacrylate scrap, the pyrolysisthereof while immersed in a stable volatile liquid solvent internal heattransfer medium consisting essentially of a crude complex of lowmolecular weight compounds of specific gravity lower than that of thescrap and which is recovered as a. residue in the purification ofmonomers by the fractional distillation of the products of pyrolyticdecomposition of such polymer scrap, said pyrolysis being conducted atan elevated temperature in the range between 200 degrees and 340 degreesC. and at reduced pressure in the range of mm. to 25 mm.

ISADOR MILLER. ARTHUR L. BEISER.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,030,901 Strain Feb. 18, 19362,248,512 Philip et al July 8, 1941 2,341,282 Marks Feb. 8, 19442,359,212 Frank et al Sept. 26, 1944 2,383,921 Soday Aug. 28, 1945

